Replied to another ancient msg I see, well my input for archives!
If regular video card troublshooting has failed....
So it's probably a heat issue, I'd get a third party heatsink/fan with the good stuff for paste. Arctic Silver 5!
I did this with my old 7950GT and it lowered the temp, significantly.
This is the heatsink I put on that and I would recommend it, but read instructions carefully!
Vantec IceBerq 6 CCB-A6C Premium VGA Cooling Kit (Retail)
I'd buy some cleaner for CPU/heatsinks also to get rid of old paste, the video card makers cut corners on that and you want the good stuff. 2 bottles one for removal of old paste, one for cleaning after that, under $10 for both, the new paste, around $6 and you don't use much, enough for thin coverage, I put it on bare handed and wash with Lava afterward.
Run GPU-Z before and after and marvel in the results!